Patent publication WO-91/08176 disclosed the technique of passing water contaminated with an halogenated hydrocarbon through a (permeable) body of a metal, for example through a body of granular iron. The body of granular iron was placed in a trench excavated in the ground in the path of an oncoming plume of the contaminated groundwater, whereby the groundwater was caused to pass through the metal. Or, the contaminated water was taken out of the ground, and passed through a container of granular iron.
Developments of that technology are disclosed in WO-92/19556 and in WO-92/19545.
Provided there is a substantial residence time, and provided that strictly reducing conditions can obtain over a prolonged period, traces of halogenated hydrocarbons in the water can be caused to break down chemically in the presence of the iron or other metal.
It is surmised that the chemical breakdown reaction may be explained as follows:
Under the conditions of the process, iron metal oxidizes to the ferrous ion, releasing two electrons, i.e.
Fe.rarw..fwdarw.Fe2++2e- PA1 H++C-Hal+2e-.rarw..fwdarw.C-H+Hal- PA1 2e-+2H.sub.2 O.rarw..fwdarw.H.sub.2 +2OH-
The halogenated hydrocarbon may be regarded as comprising a carbon-halogen component, C-Hal, and a hydrogen ion. Upon interacting with the electrons, the carbon reacts with the hydrogen ion to form a relatively non-hazardous hydrocarbon, such as methane gas, and halogen ion in solution, e.g chloride etc.
Thus, the halogenated hydrocarbon breaks down in the presence of iron, under reducing conditions.
However, what can also happen is that electrons available from the iron could, under reducing conditions, cause the surrounding water to dissociate, i.e
The H.sub.2 bubbles off as hydrogen gas, but the presence of the 2OH- serves to raise the pH of the water, which can rise high enough, say to 9 or 10, that dissolved inorganic species present in the water, which precipitate out of solution at high pH, can start to do so. At high pH, for example, carbonates of various kinds, which are nearly always present in groundwater, can precipitate.
Under high pH conditions (i.e a pH of 9 or 10) the ferrous ions, if plentiful, could combine with the dissolved substances, and ferrous carbonate or ferrous hydroxide may precipitate.
The precipitates are a problem for the process of decomposition of halogenated hydrocarbons because they tend to become deposited in the pore spaces of the body of granular iron, and to clog up the body, making the body not so permeable to the flow of groundwater.
The invention is aimed generally at promoting the breakdown of the halogenated hydrocarbon. The invention, in a particular aspect, is aimed at inhibiting the precipitation of the iron species and other substances from solution, which, if permitted, might reduce the permeability of the body of granular iron, and might coat the particles of iron with substances that would impede the reduction of the halogenated contaminant.